Electronic – GHz class sampling oscilloscopes on the cheap

I was recently perusing Picoscope's line up of sampling oscilloscopes, as they are all way outside my price range, I'd like to have a go at putting one together as a "hobby" project. ("hobby" herein meaning non-commercial, not breadboards and duct tape)

I would like to know from someone more well versed in this area if my idea is even feasible before I go spending $500+ on components.

The heart of the system revolves around a TI THS788 (a glorified 4ch stopwatch with an LSB of 13pS and a dedicated trigger input). Basically, the front end consists of several really fast comparators (most likely the HMC674LP3E). You set your trigger level on one comparator, which when triggered, starts the THS788 counting (or really sets 'zero time' as it's a free running timer).

The other 4 channels each have their own comparators and associated reference DACs, you set each comparator to trigger on a slightly different voltage and the THS788 records the time delay from the trigger. (with all differential signal lines properly length matched of course, we're dealing with picosecond delays after all)

By repeating this process over and over with slightly different setpoint voltages on the input comparators, you can build up a picture of the waveform (or at least it's rising edge). As the THS788 has 13pS time granularity, this would in theory give you an Equivalent time sampling rate on the order of 75Gsps (and the input bandwidth of ~10GHz on the comparators helps too). That'd allow for some pretty decent TDR measurements and/or eye diagrams provided that a fast enough pulse generator was available.

So should I dust off my box of K connectors and 40Gig semi-rigid or am
I getting ahead of myself?

^{simulate this circuit – Schematic created using CircuitLab}